1. Field of the Invention
The present invention relates generally to a power conversion circuit and more particularly to current monitoring in switching regulators.
2. Description of the Related Art
A power conversion circuit (e.g., a switching regulator) accepts a Direct Current (DC) voltage source at one level and outputs a desired DC voltage at another level. The switching regulator includes one or more semiconductor switches. The switches alternate between connecting and disconnecting the voltage source to circuits that drive the output. The output voltage level is related to the duty cycle of the switching. The switching is typically controlled by a Pulse-Width Modulation (PWM) circuit.
A switching regulator (or switched mode power supply) can be configured for voltage-mode operation or current-mode operation. In voltage-mode operation, an output voltage of the switching regulator is monitored to adjust switching duty cycle. In current-mode operation, the output voltage of the switching regulator and an inductor current are both monitored to adjust the switching duty cycle. The transient correction performance or compensation of a current-mode switching regulator is typically superior to a voltage-mode switching regulator.
A design challenge for the current-mode switching regulator is how to reliably monitor the inductor current. One common technique monitors a voltage across a small sense resistor coupled in series with a switch or an inductor. This common technique decreases power efficiency because of additional conversion loss due to the small sense resistor. A second technique uses a current-mirror circuit to monitor current conducted by the switch. The second technique is not possible when the switch and the current-mirror circuit are not parts of a common integrated circuit. A third technique monitors a voltage across the inductor and isolates contribution of the voltage due to current through the inductor's DC resistance. The third technique is difficult to implement because the inductor's DC resistance needs to be reliably constant and known apriori. Finally, a fourth technique monitors a voltage across the switch when the switch is on and divides the voltage by the switch's on-resistance to extract the inductor current. The main disadvantage of the fourth technique is that the switch's on-resistance typically varies significantly depending on manufacturing variation, temperature variation, and switch control variation.